The present invention relates to digital communication systems and, more specifically, to a novel method and apparatus for synthesizing the modulated carrier in a digital communication system, in order to reduce interchannel interference therein.
Electronic communications systems commonly make use of a carrier frequency which is modulated by the information to be transmitted. For example, information may be transmitted over power lines between a central facility and a multiplicity of remote locations, each tied to the power distribution network. Powerline communications systems of this type are disclosed and claimed in U.S. Pat. No. 3,973,240, issued Aug. 3, 1976; U.S. Pat. No. 3,973,087, issued Aug. 3, 1976; U.S. Pat. No. 3,944,723, issued Mar. 16, 1976; U.S. Pat. No. 4,135,181, issued Jan. 16, 1979; and U.S. Pat. No. 4,161,720, issued July 17, 1979, all assigned to the assignee of the present application and all incorporated herein by reference in their entirety.
It is also known that when a single frequency carrier is modulated, the resultant waveform contains many new frequencies, both higher and lower in frequency than the carrier, which new frequencies are called sidebands. The higher the modulation frequency, the wider the frequency spectrum occupied by the sidebands. In frequency multiplex systems, where a number of carriers are utilized in a common communications channel or medium, the sideband frequencies of one modulated carrier can overlap the frequency space allotted to an adjacent carrier and cause interference. This effect is an important factor in determining the required carrier frequency separation and the allowable information bandwidth. It is known that the signal-to-noise ratio of the received signal can be greatly improved if the frequencies at which the plurality of simultaneous carrier signals are transmitted are all odd multiples of a common frequency, such as the first sub-harmonic of the powerline frequency (60 Hertz(Hz.) in the United States) in powerline communications applications. Use of transmitted carriers at odd multiples of the powerline frequency first sub-harmonic, for improvement of interference suppression, are described and claimed in U.S. Pat. No. 4,101,834, issued July 18, 1978 and U.S. Pat. No. 4,109,204, issued Aug. 22, 1978, both assigned to the assignee of the present invention and incorporated herein in their entirety by reference. In systems of this type, where the powerlines themselves are the communications medium, the carrier frequencies may be in the 5 KHz. to 100 KHz. range. It is known that the signal to noise ratio is improved by transmitting data, in digital form, at a bit rate equal to a selected sub-harmonic of the system-wide (powerline) frequency; thus, the greatest data rate is achieved by setting the bit rate to be numerically equal to the first sub-harmonic of the powerline frequency, e.g. 30 bits per second.
It is known that a convenient and relatively inexpensive method of modulation is to utilize the so-called 180.degree. differential phase shift modulation, wherein the carrier is modulated by 180.degree. between one bit time interval and the next bit time interval to represent a binary "one" and the absence of a phase shift between one bit time interval and the next bit time interval represents the transmission of a binary "zero" data bit. Utilizing this modulation method, with an abrupt change of phase whenever a binary "one" is to be transmitted, the modulating signal with a string of binary "one"s is essentially a carrier having a modulation envelope which is essentially a 15 Hz. square wave; the modulated signal is thus very rich in harmonics and produces a wide spectrum of sidebands. As typical carrier-to-carrier separations are multiples of the common, system-wide frequency (the 60 Hz. powerline frequency) minimum carrier separation of about 7 or 8 times the system wide frequency may be desirable. This minimum carrier separation, in the range of 420 Hz. to 540 Hz., still allows the sidebands of one modulated carrier to have relatively great amplitude at the frequency of an adjacent-channel modulated carrier; for example, a sideband component falling in an adjacent channel 540 Hz. removed from a particular modulated carrier will be only 1/57 times as large as the unmodulated carrier (i.e. the adjacent channel undesired sidebands are only -35 dB. down at the desired channel). This level of interchannel interference is much greater than can be tolerated in a low error-rate system.
The spectrum produced by modulation of a carrier by prior art modulation methods is much wider than is required for accurate transmission of the modulating data bit pattern. It has been proposed that the spectrum may be reduced by filtering the modulating waveform before it is input to a linear transmitter, which translates the modulated carrier over the common transmission medium. This proposed method has at least two major problems: the generation of the carrier frequency, particularly where that carrier frequency must be synchronized to a varying local reference, such as the frequency of the A.C. waveform on the powerline, which requires an additional frequency-tracking carrier generator apparatus, such as a phase-locked loop synchronized to the powerline frequency; and the required bandpass filter must be relatively sharp and therefore have several high-Q sections, making the filter relatively expensive to manufacture and to adjust to the proper frequency, with a different filter design being required for each of the plurality of different carrier frequencies in a frequency-multiplexed system. It is therefore extremely desirable to provide a method in apparatus for providing a modulated carrier, in a digital communication system, which not only reduces interchannel interference, but which also does so in a cost-effective manner.